According to German Offenlegungsschrift No. 2,031,292, it is known to produce non-cellular, homogeneous molded polyurethane articles by reacting an organic polyisocyanate with an approximately stoichiometric quantity of an organic polyhydroxyl compound. The reaction mixture should be substantially anhydrous and should not contain more than a small amount of compounds which liberate carbon dioxide in their reactions with the polyisocyanate. It is also known from the said prior publication that the molded polyurethane articles can be obtained as either soft elastic or hard products by suitable choice of the polyhydroxyl compounds and/or polyisocyanates.
One disadvantage of this process is that the non-cellular molded products must be left inside the mold for up to 45 minutes at temperature of about 100.degree. C. to complete the chemical reaction by a heat treatment.
One fundamentally new method of obtaining non-cellular, unfoamed molded articles based on polyisocyanates has been indicated in an earlier application by one of the present applicants, U.S. Ser. No. 709,840. According to the process described in this earlier application, non-cellular molded articles can be produced in very short operating cycles. In contrast to the prior art processes mentioned above, a larger than stoichiometric quantity of isocyanate is used and, at the same time, additives which effect trimerization of the isocyanate groups present in excess of the stoichiometric quantity are included. Outstanding physical and thermal resin properties are obtained by adjusting particular combinations of starting materials so that they contain the isocyanate groups and groups which are reactive with isocyanate groups in particular equivalent proportions to each other. Different properties and qualities can only be obtained by using different combinations of starting materials and adjusting them to the optimum equivalent proportions.